DESCRIPTION: (provided by application) The overall goal of this proposal is to investigate the impact of age on skeletal muscle plasticity in response to progressive resistance training, maintenance resistance training, and detraining. The purpose is not only to characterize the effect of age on the transient nature of training adaptations, but also to identify the minimum amount of weekly loading necessary for older adults to maintain clinically important gains achieved during progressive training. The initial aim of this proposal is to test the hypothesis that, consequent to progressive resistance training, expression/availability of muscle insulin-like growth factor-I (IGF-I) and the myogenic/mitogenic actions downstream from ligand-receptor binding will increase in both young (20-30 years of age) and older (60-75 years of age) adults and will be associated with similar relative increases in the net rate of muscle protein synthesis by week 8, and similar relative increases in myofiber size, thigh lean mass, and strength performance by week 16. It is recognized, however, that structural and biological differences between young and older muscle including myofiber necrosis, preferential type II myofiber atrophy, increased noncontractile tissue, and decreased satellite cell function in older muscle may affect the absolute magnitudes of change. We expect similar relative changes. The second aim of this proposal is to test the hypothesis that reversibility of resistance training adaptations during detraining occurs more rapidly in older men and women than in young subjects. Myofiber size, thigh lean mass, strength performance, and weight-bearing exercise difficulty are expected to return to pretraining levels in older but not young subjects after 16-32 weeks of detraining. Greater spontaneous weight-bearing physical activity and higher basal levels of circulating anabolic hormones are expected to be associated with a relatively higher resistance to detraining in young subjects. Muscle IGF-I system components will also be assessed during detraining at 0, 8, 16, and 32 wk. The third aim of this proposal is to test the hypothesis that the minimum weekly volume of resistance exercise required to maintain training adaptations is greater in older adults than in young adults. Reducing weekly training volume to 1/3 of that employed during the 16-week training program is expected to maintain training adaptations in both young and older subjects, but reducing weekly volume to 1/9 of the initial program is expected to maintain training adaptations in young subjects only. This novel series of experiments will: 1) support or refute the hypothesis that local expression/availability of IGF-I in muscle is associated with mechanical load-induced changes in muscle protein synthesis and myofiber size; 2) answer the clinically significant question of whether or not resistance training adaptations are more transient in aged muscle; and 3) answer the clinically significant question of whether or not maintenance of resistance training adaptations with reduced-volume training is influenced by age.